Wednesday, January 18, 2012
Tools of the Trade: Barometer Basics
Working barometers tend to look more like thermometers than clocks, with the liquid mercury falling or rising according to the effects of wind or humidity; or both. As Peter H. Spectre wisely notes in A Mariner’s Miscellany, the words and even the number the hand on modern, decorative barometers is pointing to should be ignored all together. “What you are interested in is the rate and direction of change in the barometer readings.”
Here, then, is a by no means all inclusive list of the general changes one might see in a functional barometer and what they indicate:
When the barometer rises, keep these factors in mind: a rise with southerly winds is a sign of fine weather. If the rise comes with winds from the north, note the air and temperature; dry, cold air will portend better weather on the way, particularly in summer, while humid air with cool temperatures is a sign of windy rain. A gradual rise in the barometer usually means the weather will stay steady; a rapid rise means change – and possibly lots of it – on the way.
For a falling barometer, the same types of modifiers apply. Increased humidity and heat is a good indicator of weather coming up from the south. If in the cooler months the air is dry and cold when the barometer falls, snow is on the way. A falling barometer after a lengthy calm spells rain and possible gales, particularly if the air is warm. If the winds are from the north as the barometer falls, plan on rain or hail in the summer, and snow or sleet in the winter. A rapidly falling barometer is a sure sign of a storm.
When the barometer is steady, fair weather can usually be counted on; this in particular if the air temperature is appropriate for the latitude and season.
Keep in mind that all wind directions are for the Northern Hemisphere, and should be reversed when in the Southern.
As noted, these are simply a few points to keep in mind. For more in depth information about seafaring tools and the nautical life in general, Spectre’s book (and his annual Book of Days series) is an invaluable resource.
Header: Weymouth Bay by John Constable c 1817